Early education about sex and relationships is key, say campaigners
The fact that children raised in homes without a dad have sex earlier is down to their genes, say US researchers.
The study tested for genetic influences as well as factors such as poverty, educational opportunities and religion.
The more genes the children shared, the more similar their ages of first intercourse regardless of whether they had an absent father or not.
A spokesman for the charity, Brook, said children needed early education to help them make informed choices.
The study published in the journal, Child Development, says several theories have been advanced about the environmental factors which influence this association between absent fathers and early sex.
One suggests that because these children observe unstable or stressed parental relationships, they learn that resources are scarce, and people untrustworthy.
This leads them to mature in such a way that they are geared towards mating rather than parenting.
Another states that because adolescents reared in single-parent households may have parents engaging in sexual behaviour with partners to whom they are not married, the children may be more likely to view non-marital sex as the norm.
And a third theory states that a single-parent family structure may encourage adolescent sexuality by reduced parental control.
In other words two parents can much more closely monitor their offspring's activities and social networks, reducing the opportunities for sex.
But this study shows these factors are not as important as genes in determining early sexual behaviour.
The researchers at the University of Oregon compared the average age of first intercourse among children whose fathers were always absent, partially absent or always present throughout childhood.
They looked at more than 1,000 cousins aged 14 and older from the American National Longitudinal Survey of Youth.
For the children whose fathers were always absent, 63.2% reported having had sex.
This compared to 52.5% of children whose fathers were sometimes absent.
And only 21% of children whose fathers were always present.
The average age of first intercourse for children whose fathers were always absent was 15.28, compared to partially fathered children at 15.36 and 16.11 for children whose fathers were present for all of their childhood.
It compared children who were related in different ways to each other, and who differed in whether they had lived with their fathers.
The more genes the children shared, the more similar their ages of first intercourse, regardless of whether or not the children had an absent father.
Genetic risk factor
Jane Mendle, professor of psychology at the University of Oregon, who led the study said: "The association between father's absence and children's sexuality is best explained by genetic influences, rather than by environmental theories alone.
"While there is clearly no such thing as a 'father absence gene', there are genetic contributions to traits in both mums and dads that increase the likelihood of earlier sexual behaviour in their children.
"These include impulsivity, substance use and abuse, argumentativeness and sensation seeking."
But Professor Mendle said her study did not have the power to discriminate conclusively between genetic and environmental factors and further research with a larger number of children would be necessary.
Simon Blake, from the sexual health charity, Brook Advisory Centre, took issue with the idea that genes were the overriding factor in early sex.
He said: "We know from research that factors associated with young people having first intercourse at a younger age are: lower educational achievements; friends and the media being the main source of information about sex education; socio-economic status; early sexual experience and the earlier age at which girls start their periods.
"All young people need access to confidential sexual health services as well as high quality education about sex and relationships from a young age.
"This gives them the skills and information to make informed choices, and the self-esteem and aspirations for themselves for the future.
"Targeted outreach work is also an effective way of reaching those more vulnerable groups."